Effect of operating condition on reverse osmosis performance
for high salinity wastewater
Lingyung Hung and Shingjiang Jessie Lue
Department of Chemical and Materials Engineering and Green Technology Research Center, Chang Gung
University, Kwei-shan 333, Taoyuan, Taiwan
(E-mail:[email protected]; [email protected])
Abstract
Water scarcity is being recognized as a future threat to human activity throughout the world.
Consequently, the effort to develop alternative water resources is actively underway. Reverse
osmosis (RO) is by far the most widespread membrane based desalination process for seawater
desalination and wastewater reclamation. The objective of this study is to remove salt from high
salinity wastewater and recycle a purified stream using an RO process. It was found that high
operating pressure and temperature were beneficial for wastewater treatment using the RO process.
The permeate flux and salt rejection were enhanced with a higher operating pressure. The water
flux was 3.7×10-5-1.6×10-5 m3 m-2 s-1 for 0.5% to 2% NaCl feed solution at a pressure of 1960 kPa.
The salt rejection was 98% to 92% under the same operating conditions. The flux was lowered by
60% as the salt concentration was increased from 0.5% to 2%. The salt concentration was reduced
to 350-700 ppm from 1-2% NaCl solutions. Overall the RO is an effective process for purifying
wastewater containing high salt concentration.
Keywords
High salinity wastewater；Reverse osmosis；Water reclaim；Salt rejection；Permeate flux
Introduction
Membrane technology has revolutionized the separation industry by providing a highly selective
and low-cost alternative to separation processes. Pressure-driven membrane separation processes
(especially reverse osmosis) are important and attractive technologies for wastewater treatment and
water recycling.1 Composite RO membranes are widely used in technologies for desalination and
wastewater treatment. The membrane performance － e.g., the permeate flux and the salts
rejection —is determined mainly by the transport properties of the dense top layer. Performance is
also highly dependent on the membrane properties, solution chemistry, and operating conditions
from both permeate flux and salt rejects. Most researches reported on RO efficiency in sea water
desalination.2 3The attempts on high salinity wastewater treatment are limited. The objective of this
work was to investigate the removal of salt from high salinity wastewater using the RO process. The
NaCl concentration effect on the feed stream and operating pressure on salt rejection and permeate
flux were determined.
Methods
The RO system used in this study consisted of a pump (2SF35SEEL, Cat Pumps) controlled with a
frequency converter. The feed water temperature was controlled using cool water through a heat
exchanger. The membrane (AG1812C, GE Osmosis) having 0.2 m2 effective area with a spiral
wound configuration was studied. NaCl solutions (2L each) were used in a module feed stream. The
system was operated in the recycle mode. The salt concentration in the samples was determined
using an Ion analyzer (IA-300, DKK-TOA Corp.). The controlled variables in this study are shown
in Table 1.
Results and Discussion
The resulting salt rejection and permeate flux (Jw) under various operating conditions are
summarized in Table 2. It shows that under the same operating pressure, the salt rejection decreased
as the feed concentration increased from 0.5% to 2% (wt basis). The permeate flux declined sharply
at higher salt concentrations because of the increase of osmotic pressure from the feed solution. At
0.5% or 1.0% salt concentration, the salt rejection and permeate flux increased with increasing
operating pressure (Table 2).
The operating pressure effect on salt rejection and permeate flux on 0-2 % NaCl aqueous solution is
shown in Figures 1 and 2. Figure 1 indicates that, as the operating pressure increased from 980 to
1960 kPa, the permeate flux increased almost linearly with respect to Δp. This can be explained by
the fact that the permeate flux is directly proportional to the net driving force (i.e. ∆p-∆π). The
solute diffusion across the membrane, however, was not affected by the applied pressure. Therefore
the permeate concentration was diluted by the higher water flux, resulting in an increase in salt
rejection (Figure 2).
Generally speaking, increasing the salt concentration will enlarge the concentration difference (∆Cs)
between the feed and permeate. It also increase the salt flux (Js) due to the higher concentration
gradient inside the membrane. Our results show that the salt flux increased from 4×10-6 kg/m2 s to
2.62×10-5 kg/m2 s as the salt concentration was increased from 0.5% to 2% at 1960 kPa.At salt
concentrations of,1.0% and 1.5%, the salt fluxes were comparable (Figure 3). Under the same
applied pressure, the permeate flux decreased from 3.70×10-5 m3/m2 s to 1.58×10-5 m3/m2 s as NaCl
increasing from 0.5% to 2%. The decreases were due to higher osmotic pressure (∆π) at a higher
salt concentration, reducing the net driving forces (∆p-∆π). It was also obsvered that the salt flux
did not show a particular trend with respect to permeate flux changes using various applied
pressures (Figure 3), indicating little flux coupling effect between the water and salt.
Conclusion
High operating pressure is beneficial for wastewater treatment using the RO process. The permeate
flux and salt rejection were enhanced with a higher operating pressure. However, the salt rejection
and permeate flux were reduced with increasing feed concentration. The rejection was decreased
from 98% to 92% when the salt concentration was increased from 0.5% to 2%. The flux was
lowered by 60% at the same increased feed concentration. The salt concentration was reduced from
350-700 ppm from 1-2% NaCl solutions. Overall, RO is an effective process for purifying
wastewater containing high salt concentrations.
Table 1. Conditions and their ranges investigated in the experiments
Experimental conditions Range
0.5-2
NaCl concentration,％
Pressure, kPa
980-1960
Temperature, °C
25
Membrane area, m2
0.2
Table 2. Effect of feed concentration and operating pressure on flux and rejection.
NaCl Conc. (％) Permeate Conc. (%) ∆p (kPa) Jw (10-5m3/m2s) Rejection (％)
0.5
0.0196
980
1.5
96.21
1.0
0.0676
980
1.2
93.07
0.0
0.0000
1960
4.9
0.5
0.0108
1960
3.7
98.40
1.0
0.0350
1960
2.7
97.03
1.5
0.0547
1960
2.1
96.91
2.0
0.1659
1960
1.6
91.81
Operating condition：temperature：25°C；time：at 60 min
Figure 1. Effect of operating pressure on flux (Temp.：25°C；Time：after 60 min).
Feed：Water (left axis) and 0.5-2% NaCl (right axis)
Figure 2. Effect of operating pressure on rejection. (Temp.：25°C；Time：after 60 min).
Figure 3. Relationship between salt (Js) and permeate flux (Jw) at 1960 kPa (filled symbols) and
other operating pressures (open symbols).
References
1
C. Fritzmann, J. Löwenberg, T. Wintgens, T. Melin (2007). State-of-the-art of reverse osmosis
desalination. Desalination 216, 1–76.
2
M.N.A. Hawlader, J.C. Ho, Chua Kok Teng. (2000). Desalination of seawater: an experiment with
RO membranes. Desalination 132, 275-280.
3
L. Li, N. Liu, B. McPherson, R. Lee. (2008). Influence of counter ions on the reverse osmosis
through MFI zeolite membranes: implications for produced water desalination. Desalination 228,
217–225.